In the past 20-25 years there has been much research pertaining to the development of edible films from natural polymers, e.g. protein and carbohydrates. These films may be used as a coating to protect foods from the environment. This includes the inward migration of oxygen, moisture, and other gases and vapors that may cause deterioration of foods. An edible coating may also prevent loss of moisture, flavors, aromas, and other food constituents to the environment due to outward migration from the food. Edible films may also be used within a food to prevent migration of substances between food components.
Edible films have been produced for centuries. They date to ancient China where large vats of soy milk were heated until a skin layer formed on the top. This skin layer was removed and dried to form a film which was later filled with food and formed into shapes.
Food coatings that used waxes and gelatin were patented in the 1800s. Edible films are used today in many applications. Fruits and vegetables are generally coated with waxes and sucrose fatty acid esters/cellulose derivatives. Meats (sausages and hams) are shaped via edible films produced from collagen. Frozen meats are kept in good quality with calcium alginate films. Waxes are used on cheeses.
These edible films are well-suited for their particular applications; however there are other applications where the use of edible films would be advantageous. The main drawback to their use is their solubility or maceration in water. Protein-based edible films are generally classified as excellent barriers to oxygen. The oxygen permeability of soy films is well below the defined value for a barrier material and is similar to that of polyvinylidene chloride and ethylene-vinyl alcohol films, both well known barriers. Generally, a barrier film must have a maximum permeability of 155 cc.multidot.25 .mu.m.sup.2 .multidot.day.multidot.atm. The oxygen permeability of soy protein films has been found to be 4.1 cc.multidot.25 .mu.m/m.sup.2 .multidot.day.multidot.atm.
One of the major limitations to the usefulness of edible proteins and, in general, cellulose ether-based films is their high permeability to water vapor and loss of mechanical integrity when placed in water. The water vapor permeability of soy protein films is 4.18.times.10.sup.5 g.multidot.25 .mu.m/m.sup.2 .multidot.day.multidot.atm. If the film is to be used with foods it must have some stability to water and water vapor. Generally, protein-based films are instable to water because of the chemical nature of proteins. Proteins are composed of amino acids linked together by amide bonds. Amide bonds are highly polar, and as such, bind water. Water acts as a plasticizer, increasing the water vapor permeability along with the permeability of other gases, and eventually disrupts the macrostructure of these films, leading to a reduction in their mechanical properties. By altering the chemical structure of these films to become less polar or more hydrophobic, the water permeability and solubility is decreased. In the past, edible film researchers have used lipids as hydrophobic additives in attempts to reduce the water vapor permeability of protein-based films as well as other natural polymeric films, e.g. cellulose ether-based films. While lipids reduce the water vapor permeability, their use is limited. Protein-lipid based films are two-phase systems due to the separation of the lipid from the protein network. In order to be effective in reducing the water vapor permeability, the lipid side must be exposed the more humid environment of the packaging system. If the protein side were exposed to high or moderate humidities, it would swell and lose mechanical integrity and, as discussed earlier, show a great increase in permeability of all gases and vapors. On the other hand, if the protein side is exposed to very low humidities, it will dry out and crack, greatly increasing permeability.
When fatty acids are used as hydrophobic additives, the fatty acid crystals migrate to the surface and can be physically brushed off, resulting in a much higher water vapor permeability. The best materials for reducing water vapor permeability, paraffin wax and beeswax, have an objectional waxy mouthfeel.
In the commercial arena, "protein plastics" refers specifically to casein plastics. There have been efforts to produce a protein-based plastic that is stable to water. Research was conducted in the early part of the nineteenth century aimed at the development of casein plastics. Resistance to water penetration was attained by post-treatment of a formed product with formaldehyde which produces crosslinks through the basic side groups of the protein molecule. The major limitation to these plastics is their relatively high moisture absorption and dimensional instability. Post-treatment was later avoided by direct molding with chemical additives such as dicyandiamide, hexamethylene-tetramine, furfural, and metallic salts which impart additional crosslinking.
Another way to produce casein plastics is by direct compression molding of acid casein acylated with acetic, propionic, or butyric anhydride. These produce protein plastics with a pronounced reduction in water uptake, but they are subsequently weak and brittle. Protein plastics were also produced with higher fatty acid chlorides ranging from caprylyl to stearoyl. These produced casein plastics that showed a pronounced reduction in water absorption, but again suffered in tensile and flexural strength reduction.
Recently, there has been renewed efforts to produce water resistant films from grafted proteins. U.S. Pat. No. 4,045,239 is directed to a thermoplastic synthetic material formed from a chemically modified protein and chemically reactive additive such as a bisacrlyamide or bismethacrylamide.
U.S. Pat. No. 5,260,396 describes a water resistant film made from grafted proteins, the grafting material being monoethylenically unsaturated monomers.